Time Duration Calculator

How many hours between common start and end times

Every row is the same single subtraction the calculator above does: convert both times to minutes since midnight and take the difference. The rows are grouped by shape: full days first, then early and late shifts, then evenings, then half days and short blocks, and finally the overnight spans. Those last five run past midnight, where a plain subtraction would give a negative number, so a day is added instead. These are elapsed clock times and do not have a break taken off. If you need breaks removed and the result multiplied by an hourly rate, the hours calculator does that part.

A span that ends earlier on the clock than it starts is treated as finishing the next day, so 11:00 pm to 7:00 am reads as 8 hours rather than minus 16.

When a span is written without am or pm

A span like "11:30 to 3:30" has no single answer. Moving either end across the am and pm line changes the result by exactly 12 hours, so the same four words are 4 hours or 16 hours depending on which occurrence of the end time is meant. Neither reading is wrong on its face; the shorter one is usually what someone means, but a night shift or an overnight journey is the longer one. The time fields in the calculator above force you to state which, either with am and pm or on a 24 hour clock, so the ambiguity cannot survive into the result.

The 12 o'clock row is the one that catches people out. 12:00 am is midnight at the start of a day and 12:00 pm is noon in the middle of it, which is the reverse of what the numbers suggest, since 12 am comes before 1 am and 12 pm comes before 1 pm. On a 24 hour clock midnight is 00:00 and noon is 12:00, with no room for the mix-up.

Minutes converted to decimal hours

Payroll, billing and timesheet systems usually want hours to two decimal places rather than hours and minutes, and the two are easy to confuse: 8 hours 30 minutes is 8.50 hours, not 8.30. Divide the minutes by 60 to convert. The quarter hours are the ones worth memorising, since 15, 30 and 45 minutes give the exact figures 0.25, 0.50 and 0.75.

Decimal hours are rounded to two places for display, so a third of an hour shows as 0.33 rather than a recurring figure. Minute values that are not a multiple of five follow the same rule: divide by 60.

Decimal hours converted back to hours and minutes

Reading a figure off a timesheet or an invoice is the same conversion run backwards: multiply the decimal by 60 to get minutes. So 1.75 hours is 1.75 times 60, which is 105 minutes, or 1 hour 45 minutes. The trap is reading the digits after the point as minutes. A line showing 8.25 hours is 8 hours 15 minutes, not 8 hours 25 minutes, and the two differ by 10 minutes of pay on every entry.

Tenths of an hour are exact whole minutes, since 0.1 of an hour is 6 minutes. Thirds are not, so 0.33 hours comes back as 20 minutes only after rounding to the nearest minute.

Rounding a duration to quarter hours, tenths or five minutes

The calculator above returns exact elapsed time, but plenty of systems will not accept it. Payroll and time clocks commonly round to the nearest quarter hour, legal and agency billing to the nearest tenth of an hour (6 minutes), and some clock terminals to the nearest 5 minutes. Each column below rounds the same minute figure to the nearest multiple, with exact halves going up. Quarter hour rounding is where the well known 7 minute rule comes from: 7 minutes past a boundary rounds down, 8 minutes rounds up.

Rounding only stays fair if it runs both ways, which is why the rule is nearest multiple rather than always down. A system that only ever rounds down shortens paid time on every shift, and UK minimum wage is worked out on the time actually worked, so consistent rounding down can push an hourly rate under it. Check your own employer or client policy for the increment they use.

Working out a duration by hand

The mistake almost everyone makes is subtracting clock times as if they were ordinary decimal numbers. 17:15 minus 08:40 is not 8.75 and it is not 9 hours 25 minutes, because minutes carry at 60 rather than at 100. Two methods avoid it.

The first is to convert both times to minutes since midnight and subtract once, which is what the calculator above does. 08:40 is 8 times 60 plus 40, so 520 minutes. 17:15 is 17 times 60 plus 15, so 1035 minutes. The difference is 515 minutes, and 515 divided by 60 is 8 remainder 35, so the answer is 8 hours 35 minutes, or 8.58 decimal hours.

The second is to count forward in two hops, which is easier in your head. Go from the start to the next whole hour, then from there to the end. 08:40 to 09:00 is 20 minutes; 09:00 to 17:15 is 8 hours 15 minutes; adding them gives 8 hours 35 minutes. Counting forward also handles a span that crosses midnight without any special rule, because you never produce a negative number to correct: 22:50 to 06:20 is 10 minutes to 23:00, then 7 hours to 06:00, then 20 minutes, which is 7 hours 30 minutes.

If you subtract column by column instead, borrow 60 and not 100. For 17:15 minus 08:40 the minutes go negative, so take an hour off the 17 and add 60 to the 15: 16:75 minus 08:40 is 8 hours 35 minutes. The same borrow is why a duration written as 8.35 in a spreadsheet is wrong when what was meant was 8 hours 35 minutes, which is 8.58 in decimal hours. Whole dates rather than clock times are a different calculation, and the days between dates calculator covers that one.